The coating of substrates, for example textile fabrics, with polyurethane systems belongs to the state of the art. A distinction is made here between water-based polyurethane dispersions and solvent-based systems.
The water-based polyurethane systems cover a large field of application and have the advantage of being able substantially to dispense with volatile organic substances. Because of their necessarily hydrophilic character, however, the coatings produced have a lower water resistance than the corresponding polyurethane coatings produced from organic solutions, because the hydrophilizing groups remain in the coating film.
If it is desired to produce coatings with a good water resistance, polyurethane systems based on organic solvents are preferable to the water-based systems. In the case of one-component polyurethanes, the film forming process is a physical process which, in contrast to two-component polyurethanes, is not accompanied by a chemical reaction.
One-component polyurethaneurea coatings based on organic solvents are greatly valued by users because of their hardness, elasticity and resistance, and are used e.g. to produce finishes on textiles. Such systems are prepared by reacting an aliphatic or aromatic diisocyanate with a linear macrodiol (polyetherdiol, polyesterdiol or polycarbonatediol) to give a prepolymer, and then adjusting it to the required molecular weight by reaction with an aliphatic diamine as chain extender. Because of their structure, made up of the hard urea segment and the soft macrodiol segment, such linear polyurethaneurea solutions, which give particularly resistant coatings, have a tendency to associate and crystallize from organic solution. A good processability and good properties of the polyurethane coating are therefore no longer guaranteed.
To prevent this crystallization, solvent mixtures are recommended in the state of the art which are now regarded as unacceptable solvents in the light of improved toxicological knowledge. For aliphatic polyurethaneureas DE-A 3 134 112 and DE-A 2 457 387 recommend solvent mixtures of aromatic hydrocarbons and alcohols, for example mixtures of toluene and isopropanol.
In the light of more recent toxicological knowledge, toluene is a dangerous substance and should no longer be used in solvent mixtures for polyurethaneureas in textile coating. The amide-containing and urea-containing solvents also conventionally used hitherto in textile coating, such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone or tetramethylurea, are likewise harmful solvents and for this reason are also unusable even though their solubilizing power is known to be good.
The use of additives that disturb crystallization, for example mixtures of different amines for extending chains, does not have the desired effect either. Moreover, this deliberate introduction of heterogeneity into the polymer disturbs its composition to the point that the desired properties of the product, namely the coating, are lost. The use of a suitable solvent system is preferable to this attempted solution to the problem.
In order to be able to make further use of one-component polyurethaneurea solutions whose properties are known to be of a good standard, it is therefore necessary to have novel, toxicologically acceptable solvent systems. However, as illustrated in the Comparative Examples, many toxicologically acceptable solvent mixtures cannot be used to prepare polyurethaneurea solutions that are stable to crystallization.